It will immediately drop to a new rank based on the battle stats of the new members, but will keep the performance bonus from the previous members' war results.
So you may initially get harder matches (if the previous members did very well) or easy matches (if the previous members lost a load) but there is no such thing as "locked out of enlisting" any more.
Edit: As an example, if you purchased that shell and moved your entire current faction into that shell, replacing the existing members, when you enlist it will immediately set you to about Silver 1, just like your current faction rank - because it has already decided that your combined strength (ignoring performance modifiers) places you in that position compared to all other enlisted factions.